This is a phase III, open-labelled, randomized, multi-center out-patient comparative study of 566C80 vs. aerosolized pentamidine, an approved treatment, for the prevention of Pneumocystis carinii pneumonia in patients with HIV infection who are at high risk of developing PCP because of a prior episode or because of severely depressed DC4+ T lymphocyte counts. The study objectives are to determine and compare the efficacy of these two agents in preventing PCP and to evaluate their toxicity. Patients (n=600) are prestratified by history of prior PCP and time from therapy for their acute episode, and are randomized to 566C80 750 mg orally daily or aerosolized pentamidine 300 mg by Respirgard II nebulizer every 4 weeks for 48 weeks. Patients are seen at least monthly for clinical and laboratory evaluation. Study endpoints are: development of PCP, death, toxicity, or discontinuation of prophylaxis for reasons other than toxicity. In this study, the potential efficacy of these two agents in preventing PCP and to evaluate their toxicity. Patients (n=600) are prestratified by history of prior PCP and time from therapy for their acute episode and are randomized to 566C80 750 mg orally daily or aerosolized pentamidine 300 mg by Respirgard II nebulizer every 4 weeks for 48 weeks. Patients are seen at least monthly for clinical and laboratory evaluation. Study endpoints are: development of PCP, death, toxicity, or discontinuation of prophylaxis for reasons other than toxicity. In this study, the potential efficacy of 566C80 for the prevention of PCP is to be evaluated. Should this agent prove to be effective, it may be preferable to aerosolized pentamidine because it is systemic, and thus likely to prevent the extrapulmonary pneumocystosis associated with pentamidine, and because it has in vitro efficacy against other protozoa, and may provide prophylaxis against cerebral toxoplasmosis, another important disease in AIDS.